[Q] Samsung Galaxy Blaze S in Europe, data works but the phone calls won't work topic

My American friend came to Finland with her T-Mobile Samsung Galaxy S Blaze. She is not an expert in technical things but she mentioned that the data (messaging with different messaging apps) works with a Finnish SIM card but the phone calls won't work at all.

I took a look at her phone but didn't find anything strange. Any clue where this issue might come from? The phone is with a stock ROM so it's neither rooted nor flashed with a custom ROM.

Thanks in advance!